1. Field of the Invention
The present invention relates to thermographic printers, such as photocopiers, telecopiers, and other plain-paper printers of the type employing a magnetic recording apparatus; the invention is more particularly directed to a magnetic recording apparatus by which images of high quality can be obtained and by which the adhesion of toner to a heating head can be completely avoided without complex construction of the apparatus.
2. Description of the Prior Art
Heretofore, magnetic recording apparatus such as shown, for example, in FIG. 1A has been proposed and which comprises a magnetic recording drum 1 onto which there is applied a thermomagnetic recording material, which varies in its magnetic characteristic properties dependent upon variation of temperature, a magnetizer 2 uniformly magnetizing the thermomagnetic recording material, a heating head array 3 for forming latent magnetic image, the array having finely separated heating resistance elements arranged in a row or a plurality of rows with the heating resistance elements being energized in response to a video signal to heat the thermomagnetic recording material to its Curie point or above so that the magnetized areas on the thermomagnetic recording material are selectively demagnetized, a developing means 4 for forming a visual image by adhering magnetic toner to the latent magnetic image formed by means of the heating head array 3 (including a built-in permanent magnet having the magnetizing direction opposite to that of the magnetizer 2 and smaller magnetizing force than that thereof), a transferring means 6 transferring the visual image formed by the development to a recording paper 5 fed from a paper feeding mechanism, the image being transferred by means of magnetizing force, a fixing means 7 for fixing the transferred image to the recording paper by either heating or pressurizing the recording paper 5 which has already been transferred, and a cleaner 8 for removing and cleaning the magnetic toner remaining on the surface of the magnetic recording drum 1 after completing the transferring step.
In the above case, a laser beam, a flash lamp, or the like, in place of the heating head array, may be utilized as heat applying means.
In this apparatus, a ferromagnetic material like CrO.sub.2 may be used for the magnetic toner, and the toner is prepared by admixing the ferromagnetic material with a black coloring material consisting of carbon black and a resin powder such as polyester, polyethylene, or the like.
In the above construction, the magnetic recording drum 1 the surface of which is uniformly magnetized in a predetermined direction by means of the magnetizer 2 rotates at a constant rate to pass under the heating head array 3. Each heating resistance element of the heating head array 3 is energized on the basis of, for example, a video signal (which can be a binary signal) for one scanning line to partially demagnetize the magnetized layer of the thermomagnetic recording medium, and then the part thus demagnetized is magnetized in the opposite direction of the predetermined direction so that a latent magnetic image is formed. When the latent magnetic image thus formed reaches the position at which the developing means 4 is located, the magnetic tone is adhered by means of the developing means 4 so that a visible image is produced. Then the visible image moves to the transferring position while the recording paper 5 is conveyed to the transferring position from the paper feeding mechanism (not shown) in response to timing of the above movement, and the visible image is transferred to the recording paper 5 by means of the transferring means 6. The visible image so transferred is fixed onto the recording paper 5 by the use of the fixing means 7. The surface of the magnetic recording drum 1 after transferring the visible image is cleaned by the cleaner 8. In the magnetic recording apparatus described above, where only one piece of recording paper is recorded, uniform magnetization is carried out repeatedly by the magnetizer 2 to prepare the forming of a following latent magnetic image. However, in the case when the same image is to be printed onto a plurality of sheets of paper, the magnetizer 2 is immobilized and operation of the heating head array 3 is stopped, while plural repetitions of the developing and transferring operations are carried out with respect to the common latent magnetic image.
In the conventional magnetic recording apparatus, however, since the magnetic recording medium is partially heated in response to a video signal at a temperature at the Curie point or above to form a latent magnetic image, magnetic materials to be used are limited. In addition, there is a fear of deformation of the magnetic recording medium because high thermal energy is partially applied thereto. In the case where a laser is utilized as a means for applying heat, high power is required. Further, when a heating head array is utilized, there is the disadvantage that its magnetic recording drum is always in contact with the head array so that any magnetic toner remaining on the surface of the magnetic recording drum adheres to the heating resistance elements, and such tone is fused by the heat thereof to result in the deterioration of the quality of the picture image. For this reason, the head array has to be cleaned periodically.
In the forming of a developed image of a rather wide black colored part (i.e., a relatively wide area wholly covered with black toner), a latent magnetic image is formed over a wide area B on the magnetic recording drum 1 by magnetic fields of the opposite direction Y to the magnetizing direction X of the magnetizer 2 as shown in FIG. 1B. In such a case, since a strong magnetic attraction force is produced on the magnetic recording drum 1, a suitable amount of magnetic toner adheres to the magnetic drum 1. On the other hand, magnetic attraction force is weakened in the central part of the area B for the reason that the magnetic flux is parallel with the surface of the magnetic drum 1 so that less magnetic toner adheres to the magnetic drum 1. For this reason, a so-called "white loophole" phenomenon occurs in the central portion of the area B.